Antiretroviral therapy (ART) for HIV infection is one of the most important and rapid advances in the history of medicine. However, while effective at limiting virus replication and slowing the course of disease, ART cannot be stopped without rapid rebound of virus and progression of disease. In addition, while AIDS is prevented in patients on ART, significant non-AIDS events such as cardiovascular disease and neurocognitive decline occur more frequently and at an earlier age in HIV-infected subjects on ART than in the general population. Finally, the epidemic is too large and the barriers to roll-out too high to effectively treat every infected individual to end the epidemic. It is therefore imperative that we understand the mechanisms of how HIV persists in the infected individual to design novel therapies to eradicate the virus or functionally cure the patient. The first step in tis understanding is the development of assays to measure the level of virus persistence. Several assays exist, but have serious limitations in terms of how much information is provided, how quickly they can be performed, how expensive they are, and how variable the measurement is. We propose here a novel method to detect HIV-infected cells at single cell resolution using flow cytometry. This method, once optimized, will provide more information, be performed more rapidly, cost less, and have less variability than other assays for the measurement of HIV reservoirs. We will optimize this assay and then apply it to understand the state of the HIV reservoir in various groups of HIV-infected subjects to evaluate whether some clinically different subjects may have different sizes, activities, or reactivation potentials of their HIV reservoirs ad also to develop a high-throughput screening assay for the evaluation of novel therapeutics to reactivate or kill the latent reservoir. This process will demonstrate the utility of this assay an highlight situations in which it can be applied as well as disseminate the assay for use by others to apply as they see fit.